For a long time use has been made in various industries of cultures of microorganisms such as bacteria or the like, which dispersed in an aqueous medium enriched to varying degrees with ionized or non-ionized minerals, feed on an organic or inorganic substrate under well-defined operating conditions (with regard to aeration, temperature, pH, etc) to effect a number of transformations by assimilation and metabolization. For example, according to a now-classic method of waste-water treatment, microorganisms are used to digest pollutants either in the form of sludges activated in fermentation basins or tanks or in the form of bacterial beds where the zoogloeas, composed of microorganisms or the like, are retained on a fixed substrate serving as a filtration bed for the water to be purified.
In this process of the bacterial bed type, it is usual to employ, as the fixed substrate, hydrated-silicate-based materials such as in particular natural or artificial pozzolanas or the like. The specificity of the species of microorganism adhering to the substrate is a function of the aqueous medium to be treated and of the nutrients the latter contains. In practice, it is desirable to maximize use of the active living mass or biomass, which needs a certain number of elements to support its growth, in particular nitrogen, phosphorus, and metals such as potassium, sodium, calcium, magnesium, manganese, etc. These elements are however not often found together in the pollutants of a given water. It is then very difficult to ensure that the various species contacting a given medium have an optimum growth rate.
Hence the problem arises of developing and having available a bacterial bed where all the species of microorganisms normally used always enjoy a maximum growth rate and where the biomass is fully retained and uniformly distributed on the filtration bed.